1. Field of the Invention
The present invention relates, in general, to a 2N-order sub-harmonic frequency modulator having a high carrier suppression ratio (N is a natural number equal to or greater than 1) and direct conversion transmitter using the frequency modulator and, more particularly, to a 2N-order sub-harmonic frequency modulator (N is a natural number equal to or greater than 1) and direct conversion transmitter using the frequency modulator, in which a local oscillator generates and uses a signal with a frequency of 1/2N of an output frequency, thus improving carrier suppression ratio characteristics, reducing power consumption, realizing a small-sized transmitter, and reducing manufacturing costs.
2. Description of the Related Art
Recently, research has been carried out to develop the one-chip solution for reducing the power consumption of a wireless communication system. A transmitter used in a conventional wireless communication system employs a superheterodyne scheme. This scheme converts a low frequency signal including actual information, such as voices or images, into an Intermediate Frequency (IF) signal, imposes the IF signal on a high frequency carrier, and transmits the carrier, which is disadvantageous in that a hardware construction is complicated and power consumption is increased.
In order to solve the disadvantages of the conventional superheterodyne scheme, a direction conversion scheme allowing a base-band signal to be up-converted to a carrier without using an intermediate frequency has been utilized. The direct conversion scheme is advantageous in that it has the lowest power consumption among transmitter schemes in a wireless communication system, realizes a small-sized transmitter, and enables a transmitter to be produced at low cost.
FIG. 1 is a view showing an example of the construction of a conventional direct conversion transmitter. As shown in FIG. 1, a conventional direct conversion transmitter 100 includes a digital modulation unit 110, a Digital/Analog (D/A) conversion unit 120, a Direct Current (DC)-correction and low pass filter unit 130, a frequency modulation unit 140 having an I-modulation unit 143 and a Q-modulation unit 146, a poly phase filter 150, a local oscillator 160, a power amplification unit 170, and an antenna 180. In such a conventional transmitter construction, the output frequency FTX of the power amplification unit 160 is equal to the frequency FLO of the local oscillator 150. Therefore, there may occur a TX output-to-Local Oscillator Injection Pulling (TXLO-IP) phenomenon, in which the output frequency of the power amplification unit 160 is applied to the local oscillator 150 to influence the local oscillator 150 during the mixing of frequencies. Therefore, it is difficult to use the transmitter of FIG. 1 for a system requiring high power output, and also even for a system requiring low power output if a leakage component is great.
Various schemes have been proposed to solve the problems. For example, a technology of preventing signal distortion is disclosed in U.S. Pat. Appl. Publication No. U.S. 2003/0045248 entitled “Direct modulation transmitter utilizing signals squaring” filed by Chominski and published in Mar. 6, 2003, in which square root values on signals are obtained and then applied to a direct conversion modulator so as to prevent signal distortion occurring at the time of direct conversion. Further, this patent describes that, since the output signal of the transmitter and the local oscillator are not operated at the same frequency, TXLO-IP is improved. However, in the patent of Chominsky, a fundamental improvement plan for TXLO-IP itself is not disclosed.
Further, a direct conversion transmitter employing a low noise architecture is disclosed in PCT International Publication No. WO 03/021718 entitled “Low noise architecture for a direct conversion transmitter” filed by Motorola, Inc and published in Mar. 13, 2003. The Motorola patent discloses the architecture of a transmitter that includes a local oscillator (LO) generating a differential input LO signal actually equal to a required RF transmission frequency, a poly phase system receiving the differential input LO signal to generate first and second phase shift differential LO signals, and a quadrature mixer mixing signals using the first and second phase shift differential LO signals to generate an RF transmission signal. However, if this scheme is used, the amount of leakage occurring in the local oscillator is reduced to some degree with respect to high power signals, but the leakage is not fully prevented.
Further, a direct-conversion modulation scheme is disclosed in U.S. Pat. Appl. Publication No. U.S. 2001/0041546 entitled “Direct-conversion modulation with reduced local oscillator leakage” filed by Kazakevich and published in Nov. 15, 2001, in which two mixers are used, a first mixer mixes a signal having a frequency of half a carrier frequency with a signal to be converted, and a second mixer mixes a signal that has a frequency of half the carrier frequency and has a phase orthogonal to a carrier phase with the output signal of the first mixer. However, this scheme is problematic in that TXLO-IP is not fully eliminated.